knowledge base/practices

In a width-scaled projection, camera height must scale with the inverse aspect ratio or a narrow viewport fills with near-field geometry

added 2026-09-12·view source

3dcameralayoutgraphics

Fact

A ground-plane renderer that scales both screen axes from the viewport width — the usual way to keep a scene from stretching when the viewport shape changes — has its nearest visible ground at

z_near = cameraDepth * cameraHeight * verticalScale / (height - horizonY)

verticalScale is proportional to width, so on a narrow, tall viewport the numerator shrinks and the denominator grows. z_near collapses, and the closest strip of ground the near plane admits expands to fill a third of the screen.

Scaling the camera height by referenceAspect / aspect cancels both terms exactly and holds z_near constant at every viewport shape.

Why it matters

The obvious fix for a stretched scene — scale both axes from one dimension — is correct and introduces this second problem in the same change, so the shape that was stretched is now merely empty. It reads as a level-of-detail or culling bug rather than a camera one, because the geometry is all there and correctly drawn; there is just a single huge polygon in front of everything.

Raising the camera is also the right answer aesthetically: a tall viewport wants a more top-down view, which is what this produces for free.

How to apply

  • Derive it rather than tuning it. Write down z_near for your projection, then solve for the camera parameter that makes it independent of aspect.
  • Clamp the result (roughly 0.85x to 2.6x the reference height) so an extreme viewport does not put the camera somewhere absurd.
  • Anything positioned at a fixed fraction of the viewport height — the player's own vehicle, a reticle on the ground — must be re-derived from the projection too, or it floats off the surface as the camera moves.
  • A near-plane cull leaves bare canvas under the closest drawn surface. The ground's edges are straight lines on a plane, so their projection is straight: extrapolate the nearest quad's edges to the bottom of the screen rather than widening the near plane.